Such a method and such a device are described in German Patent Application No. 199 33 147.2. Therein, the saltwater is introduced under a first pressure into a pressure-compensation device and introduced from the pressure-compensation device, under a second, higher pressure, into a membrane module, desalted water and concentrated saltwater being removed from the membrane module. Desalted water is to be understood here as water with a salt content reduced in relation to the saltwater introduced into the device. To increase the efficiency and thus the energy balance for such a method and such a device, it is proposed there that the concentrated saltwater removed from the membrane module be introduced continuously under the second pressure into the pressure-compensation device, and be used there to apply the second pressure to the saltwater introduced into the pressure-compensation device and to introduce the saltwater out of the membrane module. Introduction of the concentrated saltwater into the pressure-compensation device is accomplished with check valves, and draining of the concentrated saltwater out of the pressure compensation device is accomplished here by means of controlled main valves. These controlled main valves are preferably actively controllable, and are arranged in the appropriate connecting lines between the membrane module and the pressure-compensation device or between the pressure-compensation device and the output for the concentrated saltwater.
A method as mentioned initially and a device as mentioned initially are known from EP 0 028 913. There, a pump is provided to compensate for pressure losses.
The recovery of energy from a highly pressurized fluid by means of a hydraulic motor driven by this fluid is known from DE 24 48 985. Cylinder/piston combinations operating in opposite phase are provided here, but they are connected mechanically by way of connecting rods to a crankshaft, which is in turn driven by a drive unit, to compensate for pressure losses. Several disadvantages are inherent to this system, however, such as a complicated bearing method and guidance of the pistons and connecting rods, since they are subjected to movements in two directions by the crankshaft. A device for desalting water by reverse osmosis, in which main valves and parallel secondary valves are provided for deriving concentrated saltwater from a membrane module and from exchange means, is known from FIG. 1 in U.S. Pat. No. 5,797,429.
A device for desalting water by reverse osmosis with two piston/cylinder devices operating in opposite phase is known from U.S. Pat. No. 6,017,200.
In known methods and devices, a high pressure is applied to the main valves. If the main valves, are operated, then a high mechanical stress results precisely at the moment of initial opening or at the last moment of closing such a main valve. However, since these main valves are designed for high flow rates, they must be correspondingly large and massive.
Since the main valves are relatively slow due to their size and mass, they are exposed to large pressure changes for a relatively long time, particularly at the beginning of the opening process and at the end of the closing process. Since such devices are intended to operate without interruption, if possible, these main valves are thus under a high long-term stress, due to the magnitude and duration of the stress on the one hand, and to the frequency of load alternations on the other.